Hebrews 12:14-15
“Follow peace with all men, and holiness, without which no one shall see the Lord.”
1. There are many things characteristic of Christianity: but more than all, and better than all, Love towards one another, and Peace. Therefore Christ also says, “My peace I give unto you.” And again, “By this shall all men know that you are My disciples, if you love one another.” Therefore Paul too says, “Follow peace with all men, and holiness,” that is, purity, “without which no man shall see the Lord.”
“Looking diligently lest any man fail of the grace of God.” As if they were traveling together on some long journey, in a large company, he says, Take heed that no man be left behind: I do not seek this only, that you should arrive yourselves, but also that you should look diligently after the others.
“Lest any man” (he says) “fail of the grace of God.” (He means the good things to come, the faith of the gospel, the best course of life: for they all are of “the Grace of God.”) Do not tell me, It is [but] one that perishes. Even for one Christ died. Have you no care for him “for whom Christ died”?
“Looking diligently,” he says, that is, searching carefully, considering, thoroughly ascertaining, as is done in the case of sick persons, and in all ways examining, thoroughly ascertaining. “Lest any root of bitterness springing up trouble you.” This is found in Deuteronomy; and he derived it from the metaphor of plants. “Lest any root of bitterness,” he says; which he said also in another place when he writes, “A little leaven leavens the whole lump.” Not for his sake alone do I wish this, he means, but also on account of the harm arising therefrom. That is to say, even if there be a root of this kind, do not suffer any shoot to come up, but let it be cut off, that it may not bear its proper fruits, that so it may not defile and pollute the others also. For, he says, “Lest any root of bitterness springing up trouble you; and by it many be defiled.”
And with good reason did he call sin “bitter”: for truly nothing is more bitter than sin, and they know it, who after they have committed it pine away under their conscience, who endure much bitterness. For being exceedingly bitter, it perverts the reasoning faculty itself. Such is the nature of what is bitter: it is unprofitable.
And well said he, “root of bitterness.” He said not, “bitter,” but “of bitterness.” For it is possible that a bitter root might bear sweet fruits; but it is not possible that a root and fountain and foundation of bitterness, should ever bear sweet fruit; for all is bitter, it has nothing sweet, all are bitter, all unpleasant, all full of hatred and abomination.
“And by this” (he says) “many be defiled.” That is, Cut off the lascivious persons.
Source: Homilies on the Epistle to the Hebrews (New Advent)